Boolean Series 键将被重新索引以匹配 DataFrame 索引

Your approach will work despite the warning, but it's best not to rely on implicit, unclear behavior.

Solution 1, make the selection of indices in a_list a boolean mask:

df[df.index.isin(a_list) & df.a_col.isnull()]

Solution 2, do it in two steps:

df2 = df.loc[a_list]
df2[df2.a_col.isnull()]

Solution 3, if you want a one-liner, use a trick found here:

df.loc[a_list].query('a_col != a_col')

The warning comes from the fact that the boolean vector df.a_col.isnull() is the length of df, while df.loc[a_list] is of the length of a_list, i.e. shorter. Therefore, some indices in df.a_col.isnull() are not in df.loc[a_list].

What pandas does is reindex the boolean series on the index of the calling dataframe. In effect, it gets from df.a_col.isnull() the values corresponding to the indices in a_list. This works, but the behavior is implicit, and could easily change in the future, so that's what the warning is about.

If you got this warning, using .loc[] instead of [] suppresses this warning.¹

df.loc[boolean_mask]           # <--------- OK
df[boolean_mask]               # <--------- warning

For the particular case in the OP, you can chain .loc[] indexers:

df.loc[a_list].loc[df['a_col'].isna()]

or chain all conditions using and inside query():

# if a_list is a list of indices of df
df.query("index in @a_list and a_col != a_col")


# if a_list is a list of values in some other column such as b_col
df.query("b_col in @a_list and a_col != a_col")

or chain all conditions using & inside [] (as in @IanS's post).

This warning occurs if

• the index of the boolean mask is not in the same order as the index of the dataframe it is filtering.

  df = pd.DataFrame({'a_col':[1, 2, np.nan]}, index=[0, 1, 2])
  m1 = pd.Series([True, False, True], index=[2, 1, 0])
  df.loc[m1]       # <--------- OK
  df[m1]           # <--------- warning
  

• the index of a boolean mask is a super set of the index of the dataframe it is filtering. For example:

  m2 = pd.Series([True, False, True, True], np.r_[df.index, 10])
  df.loc[m2]       # <--------- OK
  df[m2]           # <--------- warning
  

^{1: If we look at the source codes of [] and loc[], literally the only difference when the index of the boolean mask is a (weak) super set of the index of the dataframe is that [] shows this warning (via _getitem_bool_array method) and loc[] does not.}